a) Field of the Invention
The invention is directed to a device for displaying a video image with a source emitting at least an intensity-modulated light bundle and to a deflecting device for deflecting the light bundle, as well as to the angle-proportional scanning of N.sub.p image points in lines over an angle .alpha..sub..rho., and to the angle-proportional scanning of the light bundle of N.sub.z lines of the video image over an angle .alpha..sub.z. The invention is further directed to a production process for a device of the kind mentioned above for which a source emitting at least an intensity-modulated light bundle and a deflecting device for angle-proportional scanning of N.sub.p image points in lines over an angle .alpha..sub..rho. and for angle-proportional scanning of N.sub.z lines of the video image over an angle .alpha..sub.z of the light bundle are provided. The invention is further directed to a method for displaying a video image in which at least one intensity-modulated light bundle is emitted from a source and is deflected by means of a deflecting device for angle-proportional scanning of N.sub.p image points in lines over an angle .alpha..sub..rho. and for angle-proportional scanning of N.sub.z lines of the picture over an angle .alpha..sub.z.
By "source" is meant hereinafter not only an individual light generator but also any combination of different light sources arranged in an optional manner.
b) Description of the Related Art
Devices of the type mentioned above are known, for example, from DE 43 24 848 C1. In devices of this kind, light bundles are deflected line-by-line in the direction of a screen corresponding to the electron beam in conventional television. A further deflection vertical to the line scanning serves to scan in the image direction.
During scanning, individual image points are illuminated on the screen, wherein the intensities of the light bundles are controlled with respect to the color and brightness of the respective illuminated image points. Three lasers of different wavelength whose intensity is suitably modulated are provided in the source for color display.
Problems arise in line deflection because it requires a very high frequency. Such deflections are commonly carried out by acoustooptical modulators or, according to DE 43 24 848 C1, with polygon mirrors. However, it is anticipated that the physical boundaries with respect to scan rates will soon be met in impending high-resolution television such as HDTV. Therefore, further developments are necessary in the field of polygon mirrors or acoustooptical modulators.
With a vertical scan rate (frame frequency or field frequency) of 50 Hz, for example, the following frequencies are required for line deflection:
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interlaced 15.625 Hz
noninterlaced 31.250 Hz HDTV interlaced 31.250 Hz HDTV noninterlaced 62.500 Hz. ______________________________________
Horizontal scan rates of about 32 kHz are achieved by high-technology polygon mirror wheels. Typical specifications for these polygon mirror wheels are a rotational frequency of 1.250 Hz using 25 facets. Such polygon mirrors and other mechanical mirror deflection systems with similar parameters already practically represent an optimum with respect to attainable dynamics, deflection angle, diameter of the light bundle, freedom from dispersion, noise, construction dimensions, media supply, and costs.
However, the limitations in the dynamics of moving-mass mechanical deflection systems impose limitations on the suitability of these systems, outstanding per se, for applications in the area of high-quality laser projection technology, especially for laser shows or planetaria.
A doubling of the deflection frequency, e.g., by means of doubling the polygon facets, is impossible because of the resulting reduction in the length of the individual facets with the diameter remaining the same. On the other hand, an increase in diameter raises the requirements for the polygon mirror considerably. A doubling the rotating frequency also imposes very exacting requirements on the drive and on the bearing support of the polygon mirror, but especially on its material characteristics, because a doubling of the rotating frequency causes the centrifugal forces to be multiplied and conventional materials cannot withstand these forces beyond a given limit and can lead to the destruction of the polygon mirror.
For these reasons, the demand for increased deflecting frequencies of mechanical deflecting devices cannot easily be met. However, these difficulties may possibly be solved by new materials and new technologies for rapid, mechanically operating light deflecting devices or nonmechanical, especially electrooptical and acoustooptical, beam deflecting devices.
A further problem is posed by the demand for low-divergence light bundles so that a suitable resolution may be achieved. For this reason, lasers are usually used for generating the light bundles in accordance with the state of the art as currently known, for which the emitted light bundle is substantially parallel. However, the limits of currently available laser outputs are quickly reached, particularly in the case of large-image projections. In this regard, it would be conceivable to guide a primary light bundle over a plurality of optical amplifiers to achieve light bundles with higher output which could be combined again in an individual beam. However, only a low and unstable light intensity is available due to coherence and temperature-dependent phase position of the emitted light bundles in the far field.
In order to solve this problem, it is proposed in DE 41 39 842 A1 to divide the video image into different partial images and to display these partial images separately, each with a laser source and associated deflecting devices.
This system is uneconomical and, in addition, has a further disadvantage. In the case of the device mentioned at the start, the picture quality depends on the distance between the projection screen and the deflecting device because of the angle-proportional deflection of parallel light bundles. The picture size changes exclusively as the distance changes, wherein the picture, however, never becomes blurred when the distance changes. This makes it possible, for example, to project images on any curved surfaces so that such devices can also be used in planetaria or for flight simulation and even for new show applications. However, this is not possible in the video devices according to DE 41 39 842 A1 because the individual partial images overlap or separate from one another when the distance changes.
The same disadvantages result from a suggestion according to U.S. Pat. No. 4,796,961 for scanning with polygon mirrors in printing technology. In this case, two laser light bundles are polarized differently and are combined by means of a polarizing beam splitter in such a way that two lines are scanned simultaneously by the same polygon mirror. While this reduces complexity compared with dividing into partial images as in DE 41 39 842 A1, the line density would be dependent on the distance between the screen and the deflecting device when applying this technique in a video device because the light bundles run parallel following the polarizing beam splitter for combining the polarized light bundles. Therefore, this scanning technique cannot be used to overcome the set of problems mentioned above with respect to a video device of the type mentioned at the start.
In a video device according to WO 95/10159, a similar technique is nevertheless used. The light beams proceeding from a laser line are projected onto the deflecting polygon mirror via a lens and deflected jointly for different lines. However, in this case, an angle-proportional line raster scan in different lines is not carried out with the polygon mirror; rather, the entire laser line is displaced in accordance with a tangent dependence over the successively deflecting facets. It is questionable whether a polygon mirror of this kind can even be manufactured since it would require a very precise grinding with respect to the tangent of the deflection angle. Further, an angle-proportional deflection in this process would not produce a homogeneous line density, so that only low-quality pictures could be achieved.
Further, the laser beams reflected by the polygon mirror must be transformed again into a series of image points, for which an additional lens is required. In order to generate the final image size, additional projection optics must be connected following this additional lens to focus on the projection screen so that the above-mentioned advantages with respect to image enlargement as the distance increases and with respect to the independence of the projection surface from shape are not given.